High temperature fuel cells employing solid electrolytes to convert the chemical energy stored in gaseous fuels to electrical energy have been reported. In the most common examples of these fuel cells, oxygen is the working medium for electrochemical conversion, moving as oxide ions across the solid electrolyte to react with gaseous fuels such as coal derived gases, H.sub.2, and CH.sub.4. Electrons consumed in the reduction of oxygen to oxide ions at the cathode side of the electrolyte are released from the oxide ions at the anode side of the electrolyte to generate current flow in an external circuit. These fuel cells operate most efficiently when the gaseous fuels are completely oxidized at the anode to CO.sub.2 and H.sub.2 O in the case of carbon containing fuels or H.sub.2 O in the case of H.sub.2. The efficiency of solid oxide fuel cells is also improved by operating them at high temperatures where ohmic losses due to the impedances of the solid electrolyte and the electrode are minimized.
The conversion of the chemical energy to electricity provided by the combustion of gaseous fuels is inherently less efficient than the direct conversion of pure carbon from coal. For example, in the case of H.sub.2 and CH.sub.4, stable H-H and C-H bonds must be broken before the energy releasing reactions can occur. Consequently, less chemical energy is available to generate electricity than in the case where carbon is oxidized directly.
Attempts have been made to use coal as a carbon source in solid oxide fuel cells. However, in these cases a separate gasification step is employed to convert the coal into a gaseous fuel prior to the combustion process. Typically, this is done by passing wet oxygen (O.sub.2 plus H.sub.2 O) over coal at elevated temperatures to generate CO and H.sub.2, and the CO is subsequently reacted with O.sub.2 to produce CO.sub.2. The combustion of coal using this intermediate conversion process is still less efficient than the direct combustion of coal. For example, the gasification process used to generate CO for electrochemical conversion is a thermal process, and therefore subject to the energy conversion limits of the Carnot cycle. In addition, the primary reaction at the anode of the fuel cell is the oxidation of CO to CO.sub.2, which yields approximately half as much chemical energy for conversion to electricity as the oxidation of C to CO.sub.2.
Despite the advantages of direct electrochemical conversion of coal to electricity, there are a number of obstacles to producing a practical fuel cell for such purposes. Chief among these obstacles are the competing temperature requirements of the solid electrolyte and the combustion reactions of the fuel cell. For example, carbon can react directly with O.sub.2 to generate either CO or CO.sub.2, with complete oxidation being favored at lower temperatures (below about 700.degree. C.). On the other hand, the impedances of the solid electrolyte and its electrodes for ionic conduction increase with decreasing temperature. For this reason, solid electrolyte fuel cells are typically run at temperatures above approximately 900.degree. C. As a result, a conventional solid oxide fuel cell could not efficiently convert coal directly to electricity since either the thermodynamic efficiency of the combustion reactions or the conductivity of the electrolyte and electrodes could not be independently optimized.
In sum, it is desirable to develop methods for the direct conversion of solid fuels such as coal into electrical energy, to eliminate the energy and efficiency costs of intermediate gasification steps. However, in order to operate efficiently, a solid fuel cell must address the problems presented by the competing temperature requirements of the electrolyte and fuel cell reactions, as well as the reaction kinetics problems of solid fuel reactants.